Mutual Interference between 3,6-Dichlorinated Carbazoles and p,p’-DDT in Gas Chromatography Mass Spectrometry Analysis

The widespread contamination of the environment by polyhalogenated carbazoles (PHCZs) has been increasingly observed during the past decade. Among numerous PHCZ congeners, 3,6-dichlorocarbazole (36-CCZ) is often among the most frequently detected with higher concentrations. Although the environmental level of legacy pesticide p,p’-DDT has been declining, it continues to be ubiquitously detected. These two compounds were found to interfere with each other during analyses using gas chromatographic (GC) couples with single- or triple-quadrupole low resolution mass spectrometry (MS or MS/MS). The base peak in mass spectra was that of m/z 235 for both compounds. In MS/MS with multiple reaction monitoring (MRM), the same transactions (235→200 and 235→165) were often used. Under the same GC operating conditions, one of the two GC columns used in this work did not resolve the two compounds at baseline. Pre-treatment using cleanup column chromatography can fractionate the sample extract, with the two compounds separated in different fractions before instrumental analyses. Possible overlaps in GC retention and similarity in MS spectra might have caused data inaccuracy for 36-CCZ as well as DDT in some published studies to date, and steps to avoid the interference should be taken into quality control protocols in future research.